1. Field of the Invention
The present invention relates to an optical information storage medium which exhibits a structural change between crystalline and amorphous structure and a change in optical properties in accordance with the thermal history of temperature rise and cooling by being subjected to laser light, this being known as a phase change optical disk.
2. Description of Related Art
An optical disk recording method which uses a laser beam is capable of high-capacity recording and, because of the non-contact nature and high speed of this method, progress is being made in practical use of this method as large-capacity memory.
Optical disks can be classified into three types: read-only type disks such as those known as Compact Discs and laser discs, "write once" type disks which can be appended to by a user, and rewritable disks which can be recorded and erased repeatedly. Write many and rewritable optical disks are used as external memory for computers, and as storage for document or image files.
There are two types of rewritable disks: a phase change type optical disk which makes use of a phase change in a recording film, and a magneto-optical disk which makes use of a change in the direction of magnetization of perpendicular magnetization film.
Of these types, because the phase change optical disk not only does not require an external magnetic field, but also enables easy overwriting, as well as having the advantage of using the same playback method as a read-only disk, it is expected that this type of disk will become the main type used for such applications as overwritable digital video disks.
In the past, there has been a known type of so-called phase change optical disk, which enabled overwriting and which used a recording film which exhibited a phase changed between crystalline and amorphous phases in response to exposure to light.
In a phase change optical disk, a laser light spot from a high-power laser responsive to the information to be recorded on the recording film was shone thereonto, thereby causing a localized rise in temperature in the recording film, this bringing about a phase change between crystalline and amorphous phases to effect recording by means of changing an optical constant thereof, which can be read as differences in reflected light intensity using a low-power laser beam when performing playback.
For example, in a phase change disk which uses a recording film having a relatively slow crystallization time, the disk is turned and a laser beam is shone onto the recording film formed on the disk, the temperature of the recording film being raised to above the melting point thereof and, after the laser light is passed therethrough, quick cooling is done to record information as an amorphous condition in that part.
When erasing, the recording film temperature is held in a temperature range within which crystallization is possible, this being a range which is above the crystallization temperature and below the melting point, for a long enough time to cause crystallization to progress, thereby causing the crystallization of the recording film.
A known method for doing this that of shining an elliptical laser beam which is elongated in the direction of movement. In the case in which new information is recording as already recorded data is being erased, this being a pseudo-overwriting system using two beams, an elliptical laser beam for the purpose of erasing is positioned as to be shone ahead of the circular recording laser bean.
In a disk which uses an information recording film capable of high-speed crystallization, a single laser beam which is collimated into a circular shape is used.
A known method for doing this is that of changing the power of a laser beam between two levels, thereby causing either crystallization or a change to the amorphous phase.
That is, by shining a laser beam from a laser having sufficient power to raise the temperature of the recording film to above the melting point, the amorphous state is almost totally present when quick cooling is done.
In contrast to this, by shining a laser beam from a laser having power such that the temperature of the recording film is above the crystallization temperature and below the melting point, the part subjected to such light is crystallized.
The recording film of a phase change optical disk is formed as a grown film of a chalcogenide material such as GeSbTe, InSbTe, InSe, InTe, AsTeGe, TeOx-GeSn, TeSeSn, SbSeBi, and BiSeGe or the like, in all cases this film being formed by means of a resistive heating vacuum deposition method, electron beam vacuum deposition method, or by sputtering or the like.
The condition of the recording film immediately after formation of the film represents a type of amorphous state, and in order to form a recording amorphous part thereof by performing recording onto the recording film, it first necessary to perform initialization processing so as to change the overall recording film to the crystallized condition. Then recording is done by changing parts of this crystallized recording film to the amorphous state.
In a phase change optical disk in the past, there has been a method proposed for mark edge recording, in which information is recorded at a recording mark edge.
A disk configuration that is suitable for mark edge recording under the assumption of use at high linear speed was proposed as having a configuration which uses a transmission-type reflective film, and a silicon reflective film in particular, (for example, in Japanese Unexamined Patent Publication (KOKAI) Nos. 4-102243, 8-77596, and 8-124218).
For use as an optical disk medium for use in low linear speed mark edge recording, an example has been proposed which uses a metallic reflective film (for example: Ishida et al, p. 70 of preprints of the Sixth Phase Change Recording Research Symposium, November, 1994).
There is also known a track and groove recording method by which recording is performed in both the depressed and protruding part or tracking guide groove form on the substrate. By using this method, it is in principle possible to achieve a two-fold increase in recording density in the track direction, this enabling the achievement of high-density recording (for example, Ohno et al, p. 114 of Fifth Phase Change Recording Research Symposium, November, 1993).
As described above, the combined use of mark edge recording and track and groove recording is effective as a method of high-density recording. In overwriting phase change recording, using the known type of recording in which an amorphous mark is recorded in a crystallized part having a high reflectivity, when performing mark edge recording of a reflectivity difference playback medium in which there is a large difference in reflectivity between a crystallized part and an amorphous part, because the medium does not have light passing through it, the absorption factor of the crystallized part is quite a bit lower than in the amorphous part, this causing the problem of not being able to suppress distortion of the recorded edge when overwriting.
To solve the above-noted problem, a method of making the difference in reflectivity between the crystalline part and the amorphous part small and setting the optical phase difference therebetween so as to be large has been proposed (for example, in Japanese Unexamined Patent Publication No. 2-73537, Japanese Unexamined Patent Publication No. 2-113451, and Japanese Unexamined Patent Publication No. 7-93804).
When this method is used, it is possible to make the absorption factor of the crystalline part approximately the same as or larger than that of the amorphous part, thereby enabling suppression of the distortion of the recording mark to a small amount when overwriting.
However, in doubled track density recording, because of the necessary to achieve a uniform signal level when performing land and groove playback, and also it is required that the optical phase difference between the crystallized part and the amorphous part should be set at around zero, by merely establishing an optical phase difference and making the absorption factor of the crystalline part the same level as or greater than that of the absorption factor of the amorphous part, the level balance between land playback and groove playback is upset, so that it is not possible to obtain a medium suitable for good recording using a combination of mark edge recording and land and groove recording.
The present invention was made to solve the above-noted problems, and has as an object the provision of a phase change type optical disk that solves the problems of a medium which uses a combination of mark edge recording and land and groove recording, and which is capable of high-density recording while maintaining overwriting characteristics for the prescribed mark edge recording.